I would encourage app developers to take a look at the network architecture of the major US carriers while determining how to give their app notifications from

I would encourage app developers to take a look at the network architecture of the major US carriers while determining how to give their app notifications from the server. In the US, the 4 major LTE cellular carriers operate IPv6 only networks with NATed IPv4 access tunneled in as an unsupported afterthought. From my testing with IPv6 enabled VOIP for inbound calls, compared to FCM with calls coming from an IPv4 only server, only the former setup would reliably get calls to a test device on the first ring. If you need very low latency push notifications, IPv6 and FCM/Apple Push Notification are needed for best performance. Thus far, inter-carrier roaming generally does not support IPv6, so you still must support legacy IPv4 to a degree.